High performance composites have achieved a prominent role in the design and construction of many products, including military and commercial aircraft, sports and industrial equipment, automotive components and the like. Composites fill the need for stiffness, strength and low weight that cannot be met by other materials.
The most widely used high performance composites are fiber-polymer composites in which fibers such as oriented carbon (graphite) fibers are embedded in a suitable polymer matrix. Potential alternatives to fiber-polymer composites are molecular composites. These are materials composed of rigid-rod polymers embedded in a flexible polymer matrix. Molecular composites offer the prospect of being more economical and easier to process than conventional fiber-polymer composites.
One rigid-rod polymer produced in the past is polyphenylene. Polyphenylene is of particular interest because the basic phenylene unit has excellent thermal and chemical stability.
One method for the preparation of polyphenylenes is the oxidative coupling of aromatic compounds, such as benzene, as disclosed in P. Kovacic, et al., Chem. Rev., 1987, 87, 357-379. This method involves a chemical oxidant such as cupric chloride and a Lewis acid catalyst, and in general results in insoluble polyphenylenes of uncertain structure. It is thought that polyphenylenes prepared by oxidation of benzene are branched and low in molecular weight. Polyphenylenes of better defined composition have been reported by J. K. Stille, Die Makromolekulare Chemie, 1972, 154, 49-61.
In the past, two technical difficulties limited the use of molecular composites. First, the composites were typically a blend of the rigid rod polymers and flexible polymer. Homogeneous blends, particularly with anything but a small weight fraction of the rigid rod polymer were very difficult to obtain. Second, rigid rod polymers of significant molecular weight were difficult to prepare. For example, in the preparation of polyparaphenylene, the growing phenylene chain becomes decreasingly soluble and precipitates from solution, terminating the polymerization reaction.
M. L. Marrocco, et al., in U.S. Pat. No. 5,227,457 ("Marrocco, et al."), which is incorporated herein by reference, discloses substituted rigid-rod (i.e. predominately 1,4 or para linked) polyphenylenes made soluble by the attachment of flexible organic side groups. The side groups are chosen for a positive interaction with solvent, but are electrically neutral (uncharged) groups. The chemical and physical properties, including thermal stability, glass transition temperature (T.sub.g), and dielectric constant (K), of substituted polyphenylenes depends on the side groups attached to the polyphenylene backbone. In the substituted polyphenylenes disclosed by Marrocco, et al., inclusion of the flexible side groups resulted in a decrease in the T.sub.g of the polymer.
For many applications, particularly in the electronics industry, it would be desirable to develop polyphenylenes having higher thermal stability and higher T.sub.g, while maintaining good solubility and processability and without increasing K. It would therefore be desirable to find side groups that can be appended to polyphenylenes which have high thermal stability, but which do not lower the T.sub.g as much as previously disclosed side groups. It would also be desirable to find side groups which contribute to the solubility of the polymer but do not increase K. In addition, it would be desirable to find side groups which are basic and therefore protonatable and also thermally stable. This latter feature would allow the polymers to be used in ion exchange applications.